1. Technical Field of the Invention
This invention is concerned with an apparatus for automatically paying out and retracting a marine seismic cable in alternate cycles.
2. Discussion of the Prior Art
In marine seismic surveying, a ship tows through the water along a line of survey, a streamer cable containing a plurality of hydrophones and a corresponding plurality of electrical conductors. Periodically, as the ship occupies each designated recording station, a sound source is fired to generate acoustic waves. The generated acoustic waves propagate downwardly through the water and into underground rock layers. Reflected from the rock layers, the acoustic waves return to the water surface where they are detected by the hydrophones. The detected signals are transmitted to and processed by suitable recording equipment.
In relatively deep water the streamer cable is towed continuously at a desired depth, usually in the range of 50 to 70 feet. At that depth, even though the streamer cable is continuously towed at a speed of six knots or more, the towing-noise level is tolerably low. The streamer cable is held at a substantially constant depth by means of pressure-activated hydrofoils or "birds". The above operational mode is defined as a continuous tow.
In very shallow water it is not practical to use the pressure-activated hydrofoils. The streamer cable is held at a constant depth by suspending it from surface bouys or other means. Under tow, the surface buoys and their associated tether lines generate an intolerable amount of noise. Accordingly when data are to be recorded by the hydrophones, the cable must lie substantially at a standstill. It is impractical, too time-consuming, and too uneconomical to stop the towing ship each time that data are to be recorded. Hence, means are provided for first reeling in part of the cable. When a data recording is to be made, the cable is released, unreels, drifts and so becomes substantially stationary in the water while the towing ship continues on its way. At the end of the recording period, the cable is again partly reeled in and the free-wheeling cycle is repeated after the fashion of a yo-yo. This operational mode is defined as an intermittent tow.
There are some known systems for implementing an intermittent towing operation. In U.S. Pat. No. 3,281,767 to Cryar, two cable-connected or hydrophones are used, one on each side of the stern of the towing ship. One cable is reeled in while the other cable is reeled out at a rate comparable to the ship's speed. Two winches, driven by electric motors are employed. By means of clutches and a gear-and-sprocket arrangement, the cable reels are reversable so as to alternately deploy one hydrophone cable and then the other hydrophone cable during successive data recording cycles.
U.S. Pat. No. 3,286,225 to Huckabay et al employs a single streamer cable. In this patent an engine continuously drives a differential having two output shafts. A first output shaft is coupled to a cable drum; a second output shaft is asssociated with a brake. To reel in the cable, the brake is applied to the second output shaft, whereupon the first shaft rotates, in accordance with well-known mechanical principles, to reel in the cable. To pay out the cable, the brake is released from the second output shaft so that the drum attached to the first shaft free-wheels, allowing the cable to drift behind the towing ship.
U.S. Pat. No. 3,359,536 to Coburn employs two streamer cables as does Cryar although Coburn does not disclose in detail the mechanization of his cable reels. The '536 patent is directed primarily to use of automatic drogues to decelerate the cable motion after release of a towing force by a winch.
In all three references, the apparatus is constrained to operate only in the intermittent towing mode. The cable(s) is continuously moving back and forth relative to the ship without pause. No provision is made to lock the cable winch or drum to operate in the continuous tow mode. Accordingly the use of those systems is restricted to shallow water work. The systems are not usable for general-purpose operations.